Reciprocity

"They were both located close together when we found them," (CNN) was said to describe the bodies of two miners found after yet another mining tragedy. This statement though, or at least the "both" was redundant. So "both" A and B were located close together. Well, it's not like you could instead find A close to B, but B isn't close to A. That just can't happen, it makes no sense.

Similarly Newton's Third Law, often stated "Every action has an equal and opposite reaction." In plainer English, this means that whenever one thing applies a force on another, it applies a force back. If you push on a wall, it pushes you back. And also, this one is usually a doozy to physics newbies, however much the Earth pulls on you (due to gravity), you pull on the Earth.

It never fails that students dispute that when I first say it, whether in a physics class, or a Gen Ed. I think this year I'm going to use the analogy of proximity (as in the above quote) to start off.

Teacher draws on the board a car, and an ant.

"How close are these to each other? Is that how close the ant is to the car, or how close the car is to the ant? Same thing? Which one could get to the other sooner? So the space is big to the ant, and small to the car, it's more significant to the ant because the ant is smaller than the car.

"Forces work the same way."

Teacher draws on the board a big circle (Earth) and a stick figure person sticking out from the side.

"Just like there was a space between the car and ant, think now that there's gravity between the Earth and the person. Just like it was silly to say that the ant or the car was closer to the other, neither the Earth nor the person exerts more gravitational force on the other. Sure it's more significant to the person than to the Earth, just like the distance was more significant to the ant than the car, but the force is the same. Saying the force is 'more significant' is actually looking at the acceleration, F=ma, Newton's Second Law. Same force, if you have a small mass (person) you have a big acceleration. If you have a big mass (Earth) you have a small acceleration.

"This all leads to Newton's Third Law: 'For every action...'"